Aspirin is not inherently harmful at standard doses for most adults, but it carries real risks that have shifted medical guidelines significantly in recent years. The U.S. Preventive Services Task Force now recommends against starting daily aspirin for heart disease prevention if you’re 60 or older, concluding it offers no net benefit in that group. For younger adults and occasional users, the risk profile is different but still worth understanding.
How Aspirin Works in Your Body
Aspirin blocks two enzymes your body uses to produce prostaglandins, chemicals involved in pain signaling, inflammation, and blood clotting. What makes aspirin unusual compared to other pain relievers is that its effects are irreversible. Once aspirin disables the clotting machinery in a platelet, that platelet stays disabled for its entire lifespan of 7 to 10 days. This is why aspirin is so effective at preventing blood clots, and also why it increases bleeding risk for over a week after you stop taking it.
The same enzyme aspirin blocks also maintains the protective mucus lining of your stomach. Shutting it down reduces pain and inflammation but leaves your stomach and intestines more vulnerable to acid damage. This tradeoff is central to most of aspirin’s harmful effects.
Gastrointestinal Bleeding
The most common serious harm from regular aspirin use is bleeding in the digestive tract. In the large ASPREE trial, which followed healthy older adults for nearly five years, daily low-dose aspirin increased overall gastrointestinal bleeding by 60% compared to placebo. Upper GI bleeds were nearly twice as common in the aspirin group. That sounds alarming, but context matters: the absolute number of bleeds was still relatively small. Out of thousands of participants, the difference was roughly 40 extra upper GI bleeds in the aspirin group over nearly five years.
Your individual risk depends on several factors. Older age, a history of ulcers, and concurrent use of other anti-inflammatory drugs or blood thinners all raise the odds substantially. Regular alcohol use compounds the problem, since alcohol also irritates the stomach lining and interferes with clotting. For a younger person taking aspirin occasionally for a headache, serious GI bleeding is uncommon. For an older adult taking it daily, the risk accumulates.
Bleeding Risk in Older Adults
Beyond the gut, aspirin’s anticlotting effect raises concerns about bleeding elsewhere in the body, particularly in the brain. In the same ASPREE trial of adults over 65, hemorrhagic stroke occurred in 0.5% of aspirin users versus 0.4% in the placebo group. While this difference was not statistically significant on its own, it reflects a pattern: in older adults, the blood-thinning properties of aspirin become increasingly risky as blood vessels become more fragile. The USPSTF’s recommendation against starting aspirin after age 60 for primary prevention is based on this calculus. The bleeding risks begin to outweigh the cardiovascular benefits when you don’t already have heart disease.
When Aspirin Still Has Clear Benefits
The risk picture changes completely if you’ve already had a heart attack or stroke. For people with established cardiovascular disease, aspirin reduces the chance of another event by about 19%. That’s a meaningful reduction, and the benefit consistently outweighs the bleeding risk in this group. If your doctor has prescribed daily aspirin after a cardiac event, the drug is working as intended and stopping it without medical guidance can be dangerous.
For adults aged 40 to 59 who have a 10% or greater chance of a cardiovascular event in the next decade, the decision is more nuanced. In this range, the potential benefit exists but must be weighed against personal bleeding risk factors. This is genuinely a case-by-case decision rather than a blanket recommendation.
Kidney Effects With Long-Term Use
Chronic aspirin use has been linked to kidney problems. A study published in the New England Journal of Medicine found that regular use of aspirin was associated with a 2.5-fold increase in the risk of chronic kidney failure. The risk rose with increasing lifetime doses. This doesn’t mean occasional aspirin use damages your kidneys, but years of daily use, especially at higher doses, can gradually impair kidney function. People who already have reduced kidney function are more vulnerable.
Aspirin and Children
Aspirin poses a unique and serious danger to children and teenagers. When given during or after a viral infection like the flu or chickenpox, aspirin can trigger Reye’s syndrome, a rare but potentially fatal condition that causes swelling in the liver and brain. In younger children, early signs include diarrhea and rapid breathing. In older kids and teens, the warning signs are persistent vomiting and unusual sleepiness. Most children who are properly diagnosed survive, but without treatment, Reye’s syndrome can cause death within days. This is why aspirin is not recommended for anyone under 18 with a fever or viral illness. Acetaminophen and ibuprofen are safer alternatives for children.
Aspirin Sensitivity and Respiratory Reactions
Some people have a specific sensitivity to aspirin that goes beyond typical side effects. A condition known as aspirin-exacerbated respiratory disease involves the combination of nasal polyps, chronic sinus inflammation, asthma, and reactions to aspirin or similar anti-inflammatory drugs. It affects less than 1% of the general population, but the prevalence jumps to 10 to 20% among people with asthma and as high as 30 to 40% among asthmatics who also have nasal polyps. Reactions can include sudden worsening of asthma symptoms, nasal congestion, and difficulty breathing. If you have asthma and notice respiratory symptoms after taking aspirin or ibuprofen, this condition may be the reason.
Drug Interactions That Increase Risk
Aspirin’s blood-thinning effect becomes more dangerous when combined with other medications that also affect clotting or irritate the stomach. Blood thinners like warfarin are the most significant concern, as the combination can dramatically increase bleeding risk. Other anti-inflammatory pain relievers like ibuprofen and naproxen overlap with aspirin’s effects on the stomach lining and clotting, compounding both risks. Alcohol is another important interaction, both because it irritates the GI tract and because it can amplify aspirin’s anticlotting effects. Corticosteroids, certain diabetes medications, and the chemotherapy drug methotrexate also interact with aspirin in ways that can increase toxicity.
Signs of Aspirin Toxicity
Taking too much aspirin, whether accidentally or intentionally, produces a recognizable set of symptoms. Mild toxicity causes ringing in the ears (tinnitus), dizziness, nausea, vomiting, and lethargy. These symptoms can occur even at doses only modestly above the therapeutic range. Severe toxicity is a medical emergency, with symptoms including high body temperature, rapid breathing, seizures, and coma. Therapeutic blood levels of salicylate range from 150 to 300 micrograms per milliliter; anything above 300 is considered toxic. In severe cases, dialysis may be needed to clear the drug from the bloodstream.
The risk of accidental overdose is higher with aspirin than many people realize, partly because salicylate appears in other products like bismuth subsalicylate (the active ingredient in some stomach remedies) and certain topical creams. Taking multiple salicylate-containing products simultaneously can push levels into the toxic range without any single product seeming excessive.